In bulk polymerization (also called mass polymerization), the reaction medium is typically solventless, i.e., the monomer is polymerized in the absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization involves only the monomer and catalyst, there is a minimum potential for contamination and the product separation is simplified. It also offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages with reduced emissions and wastewater pollution.
Nonetheless, bulk polymerization requires very careful temperature control, and there is also the need for strong and elaborate stirring equipment since the viscosity of the polymerization system can become very high. In the absence of added diluent, the cement viscosity and exotherm effects can make temperature control very difficult. Also, cis-1,4-polybutadiene is insoluble in 1,3-butadiene monomer at elevated temperatures. It is therefore preferred to operate bulk polymerization at a low temperature.
Catalyst systems comprising a lanthanide compound, an alkylating agent, and a halogen source are useful for polymerizing conjugated diene monomers. They are highly stereospecific and can produce conjugated diene polymers having high cis-1,4-linkage contents. The resulting cis-1,4-polydienes have a linear backbone structure, exhibit good green strength, and have excellent viscoelastic properties. The linear backbone structure is believed to provide better tensile properties, higher abrasion resistance, lower hysteresis loss, and outstanding fatigue resistance in rubber compounds. Therefore, these cis-1,4-polydienes are particularly suitable for use in tire components such as sidewall and tread.
Commercially useful lanthanide catalyst systems are three-component catalyst systems that comprise a neodymium carboxylate as the lanthanide compound, either a trialkylaluminum or a dialkylaluminum hydride as the alkylating agent, and an alkylaluminum halide as the halogen source. The molecular weight of cis-1,4-polybutadiene produced by these catalysts is mainly influenced by the polymerization temperature, the monomer concentration, the catalyst concentration, and the ratio of the organoaluminum compound to the lanthanide compound. The molecular weight increases with lower polymerization temperature, higher monomer concentration, lower catalyst concentration, and lower ratio of the organoaluminum compound to the lanthanide compound.
Despite many advantages of the lanthanide-based catalysts, when they are employed in bulk polymerization of 1,3-butadiene, the low polymerization temperature and the high monomer concentration combine to give cis-1,4-polybutadiene having an excessively high molecular weight, which gives an excessively high Mooney viscosity and makes processing difficult.
One approach to reducing the molecular weight of cis-1,4-polybutadiene produced with the lanthanide-based catalyst systems is to employ a very high catalyst concentration or a very high ratio of the organoaluminum compound to the lanthanide compound, both of which result in very high catalyst costs. The use of high catalyst levels also necessitates the removal of catalyst residues from the polymer, yet this removal (also referred to as de-ashing) is time-consuming and adds cost.
The molecular weight of cis-1,4-polybutadiene produced with the lanthanide-based catalyst systems can also be reduced by reducing monomer conversion, because these catalyst systems display pseudo-living behavior so that molecular weight increases with monomer conversion. Unfortunately, reducing monomer conversion decreases productivity. In addition, when the monomer conversion is reduced, the amount of polymer produced with a specified amount of catalyst is also reduced.
The use of dialkylaluminum hydride instead of a trialkylaluminum as the alkylating agent also reduces the molecular weight of cis-1,4-polybutadiene produced with the lanthanide-based catalysts. Dialkylaluminum hydrides are better chain transfer agents than trialkylaluminums, and therefore less dialkylaluminum hydride—as compared to trialkylaluminum—is required to obtain a target molecular weight. Therefore, the use of dialkylaluminum hydrides reduces cost. But, the use of dialkylaluminum hydrides in low-temperature bulk polymerization has serious drawbacks. At low temperatures, dialkylaluminum hydrides are known to exist in oligomeric (such as trimeric) forms, which dissociate to the monomeric form only at elevated temperatures. The oligomeric structures of dialkylaluminum hydrides causes low catalyst activity. And, the resulting polymer has a very broad molecular weight distribution and contains a fraction of ultrahigh molecular weight material, which impacts processing and viscoelastic properties. The broad molecular weight distribution generally results in higher hysteresis loss in rubber vulcanizates. The ultrahigh molecular weight fraction causes high compound Mooney viscosity and high solution viscosity. The high compound Mooney viscosity adversely affects the processability and scorch safety of rubber compounds, and the high solution viscosity is disadvantageous if the cis-1,4-polybutadiene is used in the production of high-impact polystyrene. Moreover, during the synthesis of cis-1,4-polybutadiene, the high solution viscosity causes difficulty in stirring and transferring the polymer cement and reduces the capacity for removing the heat of polymerization, which limits the polymer concentration that can be achieved in production.
Therefore, there is a need to develop an improved bulk polymerization process that utilizes a lanthanide-based catalyst system for producing cis-1,4-polydiene having commercially desirable Mooney viscosities without having to employ high catalyst levels.